1. Field of the Invention
The present invention relates to a driver circuit of a display device and a display device using the driver circuit. In particular, the invention relates to a technique for correcting luminance of a light-emitting element.
2. Description of the Related Art
As a monitor of a television receiver or a personal computer, demand for a thin display has been rapidly increasing, and further development thereof has been advanced. As a typical example of the thin display, a liquid crystal display is known. In recent years, a display utilizing an electroluminescence element (hereinafter also referred to as an ‘EL element’) is also developed. Such a display utilizing an EL element has the advantages of a thin shape, light weight and high image quality as well as high response speed and wide viewing angle. Therefore, it is expected as a next-generation display.
However, the EL element using an organic material has a problem in that the resistance thereof changes with time, which leads to a decrease in the light-emission efficiency. Further, it has another problem in that the resistance thereof changes in accordance with changes in the ambient temperature of the EL element. In order to solve such problems, a display provided with a monitoring element has been developed (e.g., see Patent Document 1). The display is provided with a monitoring element having a common cathode to an EL element in a pixel portion, and a constant current is supplied to the monitoring element so that a voltage value of an anode of the monitoring element is sampled. By using the sampled voltage value as an anode voltage of the EL element in the pixel portion, a current value of the EL element can be constant even when the resistance value thereof has changed, thereby a difference between the actual current and the desired current can be minimized. As a driving method, a digital time gray scale method is adopted.
A display shown in FIG. 2 is provided with an external circuit 2004 and a panel 2010. The external circuit 2004 includes a constant current source circuit 2001, a power source 2002 and a signal generator 2003, which is connected to the panel 2010 through a flexible printed wiring (FPC) connecting portion 2005. The panel 2010 includes a signal line driver circuit 2006, a scan line driver circuit 2007, a pixel portion 2009 provided with an EL element 2011 and a monitoring element portion 2012 over a substrate 2008.
The power source 2002 generates power having desired voltage values based on the power supplied from a battery or an AC power source, and supplies the power to various circuits incorporated in the display. The signal generator 2003 receives power, video signals, synchronous signals and the like, and generates clock signals for driving the signal line driver circuit 2006 and the scan line driver circuit 2007 as well as converting various signals. The EL element 2011 in each pixel is controlled to emit light or no light with a digital video signal from the signal line driver circuit 2006 and a selection pulse from the scan line driver circuit 2007. The constant current source circuit 2001 supplies a desired current value to the monitoring element portion 2012, and a potential sampled at an anode portion of the monitoring element portion 2012 is used as an anode potential of the EL element in the pixel portion 2009.
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-333861
However, when the constant current source circuit 2001 for supplying a constant current to the monitoring element portion 2012 is formed with thin film transistors (hereinafter also referred to as ‘TFTs’) over the substrate 2008, the current value supplied from the constant current source circuit 2001 fluctuates due to characteristic variations of TFTs in each production lot or each panel. Furthermore, the current value supplied from the constant current source circuit 2001 is required to be set by taking into consideration the variations in the film deposition of EL elements. In order to control the output value of the constant current source circuit 2001, a larger number of components is required, which is disadvantageous.